In general, an electro-galvanized steel sheet is widely used in such areas as home electrical appliances and automotive outer shell because of its excellent corrosion resistance.
Recently, however, there is an increasing demand from users and manufacturers of said electro-galvanized steel sheet for the improvement of corrosion resistance of the electro-galvanized layer of an electro-galvanized steel sheet with a view to simplifying the manufacturing process of electro-galvanized steel sheets, saving zinc resources, and reducing the manufacturing costs.
The following methods are known as those for improving bare corrosion resistance of an electro-galvanized steel sheet with a view to improve a corrosion resistance of an electro-galvanized steel sheet:
(a) in an acid electro-galvanizing bath added with from 5 to 50 g/l metallic Co in the form of at least one of water-soluble compounds of Co, subjecting a steel sheet to an electro-galvanizing treatment so that the electro-galvanizing layer contains Co compounds; PA1 (b) in an acidic electro-galvanizing bath added with from 0.3 to 20 g/l metallic Co in the form of at least one of water-soluble compounds of Co and also added with at least one of water-soluble compounds of molybdenum, tungsten or iron, subjecting a steel sheet to an electro-galvanizing treatment so that the electro-galvanized layer contains compounds of said metals. PA1 (a) a metal layer formed on the surface of a steel sheet by an electro-galvanizing treatment, said layer containing Zn, as the main constituent, and at least one of Mo, W and Co, as the sub-constituent, in an amount of from 0.05 to 7 wt.% as at least one of metallic Mo, W and Co relative to the total weight of the electro-galvanized layer in the form of at least one of the oxides of Mo, W or Co; PA1 (b) a metal layer formed on the surface of the steel sheet by an electro-galvanized treatment, said layer containing Zn, as the main constituent, and at least one of Mo, W and Co, as the sub-constituent, in an amount of from 0.05 to 7 wt.% as at least one of metallic Mo, W and Co relative to the total weight of the electro-galvanized layer in the form of at least one of the oxides of Mo, W or Co, and also containing at least one of Fe, Ni, Sn, and Pb, as the further sub-constituent, in an amount of from 0.5 to 15 wt.% as at least one of metallic Fe, Ni, Sn and Pb relative to the total weight of the electro-galvanized layer in the form of at least one of metals or compounds of Fe, Ni, Sn or Pb. PA1 in an acidic electro-galvanizing bath principally comprising Zn ion, and containing: PA1 at least one additive selected from the group consisting of: PA1 subjecting a steel sheet to an electro-galvanizing treatment to form a first galvanized layer on the surface thereof; and then, subjecting said electro-galvanized steel sheet having said first galvanized layer thus formed to an ordinary chromating treatment. PA1 (a) Chemical composition of the acidic electro-galvanizing bath employed: PA1 (b) Electro-galvanizing conditions: PA1 moving a steel strip in an acidic electro-galvanizing bath containing cobalt and chromium in parallel with the plane of at least one anode plate to subject said steel strip to an electro-galvanizing treatment, thereby forming, on at least one surface of said steel strip, an electro-galvanized layer excellent in bare corrosion resistance and corrosion resistance after chromating;
According to the above-mentioned prior arts (1) and (2), it is possible to improve bare corrosion resistance as compared with that of the pure-zinc galvanized steel sheet.
However, in view of the recent demand for an electro-galvanized steel sheet further excellent in bare corrosion resistance, the electro-galvanized steel sheets based on the prior arts disclosed in the above-mentioned prior arts are still insufficient in some cases in bare corrosion resistance. In addition, when the electro-galvanized layer is subjected to a chromating treatment according to the above-mentioned prior arts (1) and (2), corrosion resistance after chromating has been inferior because of the insufficient weight of chromium deposited onto the surface of the electro-galvanized steel sheet.
Therefore, the present inventors proposed previously the following method, with a view to solving the problems involved in the above-mentioned prior arts (1) and (2):
and further containing:
It is sure that, according to the above-mentioned method, bare corrosion resistance of an electro-galvanized steel sheet is improved over those of the prior arts (1) and (2) previously described, and at the same time, corrosion resistance after chromating treatment is remarkably improved.
In the method of the above item (3), however, when changing the galvanizing current density so as to meet a change of the line speed or other conditions, the Co content of the electro-galvanized layer also changed, and it was impossible to obtain an electro-galvanized steel sheet having an electro-galvanized layer which has a uniform external appearance and was excellent in bare corrosion resistance.
FIG. 1 illustrates the relationship between the galvanizing current density when subjecting a steel sheet to an electro-galvanizing treatment and the Co content of the electro-galvanized layer.
The test conditions for this were as follows:
As is clear from FIG. 1, a change of the galvanizing current density causes a large change of the Co content of the electro-galvanized layer.
For this reason, there is a demand for a method for manufacturing an electro-galvanized steel strip, capable of forming an electro-galvanized layer of which the external appearance does not become irregular even at a change of the galvanizing current density and which shows a stable bare corrosion resistance and is excellent in corrosion resistance after chromating treatment. However, such a method has not yet been proposed.